Do not abort connection after encoding error in postgresql

[G1gg1L3s]

This PR implement skipping postgres packets from database after encoding error, almost in the same way as the previous PR does for mysql. It also adds a bunch of tests for different cases:

• Simple queries
• Extended queries (and prepared statements, that are basically the same under the hood).
• Batched extended queries (when database sends n packets and waits for the next request)

The only issue with postgres is that a state of transaction is stored at database side, and changes in case of some errors (even parsing ones). When the state is changed, commit is basically rollback:

test=# begin;
BEGIN
test=*# select 1/0;
ERROR:  division by zero
test=!# commit;
ROLLBACK

But our error will not trigger that:

test=# begin;
BEGIN
test=*# select value_bytes from testtype;
ERROR:  encoding error in column "value_bytes"
test=# commit;
COMMIT

So, for now I will mention it in the docs. Currently there is no straight and simple solution.

P.S. test.py crossed the milestone of 10 thousands lines, so my congratulations! It takes 5 second for my formatter to proceed the file. That's what I call success.

Checklist

• [x] Change is covered by automated tests
• [x] The coding guidelines are followed
• [x] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [x] ~CHANGELOG.md is updated (in case of notable or breaking changes)~
• [x] ~CHANGELOG_DEV.md is updated~
• [x] ~Benchmark results are attached (if applicable)~
• [x] ~Example projects and code samples are up-to-date (in case of API changes)~